enic: handle error condition properly in enic_rq_indicate_buf

In case of error in rx path, we free the buf->os_buf but we do not make it NULL.
In next iteration we use the skb which is already freed. This causes the
following crash.

[  886.154772] general protection fault: 0000 [#1] PREEMPT SMP
[  886.154851] Modules linked in: rpcsec_gss_krb5 auth_rpcgss oid_registry nfsv4 microcode evdev cirrus ttm drm_kms_helper drm enic syscopyarea sysfillrect sysimgblt psmouse i2c_piix4 serio_raw pcspkr i2c_core nfs lockd grace sunrpc fscache ext4 crc16 mbcache jbd2 sd_mod crc_t10dif crct10dif_common ata_generic ata_piix virtio_balloon libata scsi_mod uhci_hcd usbcore virtio_pci virtio_ring virtio usb_common
[  886.155199] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W      3.17.0-netnext-05668-g876bc7f #272
[  886.155263] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[  886.155304] task: ffffffff81a1d580 ti: ffffffff81a00000 task.ti: ffffffff81a00000
[  886.155356] RIP: 0010:[<ffffffff81384030>]  [<ffffffff81384030>] kfree_skb_list+0x10/0x30
[  886.155418] RSP: 0018:ffff880210603d48  EFLAGS: 00010206
[  886.155456] RAX: 0000000000000020 RBX: 0000000000000000 RCX: 0000000000000000
[  886.155504] RDX: 0000000000000000 RSI: 0000000000000001 RDI: 004500084e000017
[  886.155553] RBP: ffff880210603d50 R08: 00000000fe13d1b6 R09: 0000000000000001
[  886.155601] R10: 0000000000000000 R11: 0000000000000000 R12: ffff880209ff2f00
[  886.155650] R13: ffff88020ac0fe40 R14: ffff880209ff2f00 R15: ffff8800da8e3a80
[  886.155699] FS:  0000000000000000(0000) GS:ffff880210600000(0000) knlGS:0000000000000000
[  886.155774] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  886.155814] CR2: 00007f0e0c925000 CR3: 0000000035e8b000 CR4: 00000000000006f0
[  886.155865] Stack:
[  886.155882]  0000000000000000 ffff880210603d78 ffffffff81383f79 ffff880209ff2f00
[  886.155942]  ffff88020b0c0b40 000000000000c000 ffff880210603d90 ffffffff81383faf
[  886.156001]  ffff880209ff2f00 ffff880210603da8 ffffffff8138406d ffff88020b1b08c0
[  886.156061] Call Trace:
[  886.156080]  <IRQ>
[  886.156095]
[  886.156112]  [<ffffffff81383f79>] skb_release_data+0xa9/0xc0
[  886.157656]  [<ffffffff81383faf>] skb_release_all+0x1f/0x30
[  886.159195]  [<ffffffff8138406d>] consume_skb+0x1d/0x40
[  886.160719]  [<ffffffff813942e5>] __dev_kfree_skb_any+0x35/0x40
[  886.162224]  [<ffffffffa02dc1d5>] enic_rq_service.constprop.47+0xe5/0x5a0 [enic]
[  886.163756]  [<ffffffffa02dc829>] enic_poll_msix_rq+0x199/0x370 [enic]
[  886.164730]  [<ffffffff81397e29>] net_rx_action+0x139/0x210
[  886.164730]  [<ffffffff8105fb2e>] __do_softirq+0x14e/0x280
[  886.164730]  [<ffffffff8105ff2e>] irq_exit+0x8e/0xb0
[  886.164730]  [<ffffffff8100fc1d>] do_IRQ+0x5d/0x100
[  886.164730]  [<ffffffff81496832>] common_interrupt+0x72/0x72

fixes: a03bb56e67c357980dae886683733dab5583dc14 ("enic: implement rx_copybreak")
Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>